This invention relates to a photographic silver halide photosensitive material and a photographic combination of a photographic silver halide photosensitive material with a radiographic intensifying screen. More particularly, it relates to a photographic silver halide photosensitive material and a photographic combination capable of forming improved images in the area of mammography of photographing soft tissue using low-energy X-rays radiated at a tube voltage of less than 40 kVp, especially forming images of consistent high quality under different service conditions.
The number of breast cancer patients is increasing in these years. Common means for the examination and diagnosis of breast cancer include touch, ultrasonography, and mammography or breast radiography. Among others, the mammography using fluorescent screens is believed more effective. With respect to the advance of breast diagnostic systems, reference should be made to the literature, for example, Phys. Med. Biol., 41 (1996), 315.
Radiography using fluorescent screens has been frequently used in the medical field for photographing chest, abdomen and stomach. Among these, the breast photographing radiography is regarded as a special one from the standpoint of developing a screen/photographic silver halide photosensitive material system therefor. More particularly, the mammography carries out photographing using low-energy X-rays radiated at a tube voltage of less than 40 kVp, which are seldom used in ordinary radiography, for the purpose of compensating for the very low contrast of a breast or subject. Also very high sharpness is required since very fine calcified images of the order of several hundred micrometers in size must be observed. Since the mammography uses a different radiation source and requires different performance than the ordinary radiographic system, different performance and design are required for its screen/photographic silver halide photosensitive material system. Engineers can little rely on the technology developed for the conventional radiography. Each manufacturer has developed and marketed a system specialized for mammography. A number of modifications have been reported.
U.S. Pat. No. 4,914,303 discloses a screen for mammography wherein stationary noise is minimized by optimizing a luminophore size distribution. Also, JP-B 18955/1995 discloses a method for producing a screen of high image quality by optimizing a dye distribution of a fluorescent layer. These proposals relate to the screen alone and their level of improvement is approximate to currently commercially available screens and still insufficient.
JP-A 179145/1989 and 45807/1993 disclose photographic silver halide photosensitive materials for mammography. These materials are still unsatisfactory. There is a demand for further improvement. As seen from Examples of these patent specifications, the photosensitive materials have a contrast of less than 3.6, suggesting that an effective photosensitive material having a high contrast sufficient for mammography has not been developed. In 1996, Eastman Kodak Company marketed a new photosensitive material MinR2000, which merely has a contrast of 3.5 falling in the range of the existing system. Also EP 0712036 discloses a novel mammographic system wherein the contrast is only 3.6 which does not surpass the prior art range.
Although engineers have sought for high contrast photosensitive material, it is technically difficult to produce a photosensitive material having a high contrast in the desired sensitivity range. According to our research, if the contrast is increased above 3.6, such a photosensitive material is inconvenient to use because the photographic density largely varies with a differential variation of a developer in a day. When one tries to increase the contrast of photosensitive material as an extension of the prior art, the graininess is exacerbated beyond the permissible range, that is, favorable results are not always obtained.
Despite the increased demand for the diagnostic efficacy of mammography, no engineers could find a new key toward further development.
Although the mammography generally uses one sheet of intensifying screen and a one-side photosensitive material, JP-B 18956/1995 discloses a double side system using two front and back intensifying screens for achieving high image quality. In this system, however, no substantial advances are found because of an unsharpening effect resulting from the crossover light between the double side screens, mixing of scattered radiation, and the too thin front intensifying screen. Several specific screens are described in Examples, all of which are intensely colored and contain a phosphor with a large particle size. Even if such a screen is used only on one side, the screen itself is not advanced in performance.
JP-A 96740/1990 discloses a system wherein a fluorescent layer and a photosensitive layer are simultaneously coated on a support. Since this system does not reuse the phosphor, it lacks reality for the screening application in that a single radiographic shot becomes expensive.
Though not designed for mammography, high sharpness one-side photosensitive material systems are disclosed in JP-A 43861/1995 and WO 93-01522. However, the system of JP-A 43861/1995 exhibits a sharpness which is insufficient for mammographic use and lacks the concept of designing a high contrast photosensitive material. The system of WO 93-01522 uses a ultraviolet emitting phosphor. Although its screen can be designed so as to exhibit a high sharpness, an extension of this system never reaches a satisfactory mammographic system.
Despite these numerous attempts, no satisfactory results have been achieved and no commercial items have been widespread.
In the area of mammography possessing the unique feature distinguishable from the ordinary radiography in that photographing is done with low-energy X-rays, the development of a high image quality system has been desired for improving the diagnostic efficacy.